1. Field of the Invention
In general, the invention relates to the field of fusion research and to the use of a plurality of laser beams travelling over different paths to impinge upon a fusion fuel pellet from different directions to initiate a fusion reaction by compressing the pellet. More particularly, the invention relates to a method and apparatus for the synchronization or timing of multiple laser beams by detecting differences in the optical path lengths of the beams and adjusting the optical path lengths to make them equal. Even more specifically, the invention relates to an optical heterodyne interferometric detection technique and apparatus for detecting differences in the optical path lengths between a master beam and a comparison beam derived from a common source beam, so that the comparison beam can be adjusted to have an optical path length substantially equal to that of the master beam whereby the beams arrive at a common target at the same time.
2. Description of the Prior Art
Optical heterodyne interferometry per se is known in the prior art; for example, this type of interferometry is described in an article by Jean-Louis Meyzonnette and N. Balasubramanian in Optical Engineering, Vol. 13, No. 5, pp. 455-459 (September-October 1974). In addition, optical heterodyne interferometric detection techniques have been used for determining phase differences of two laser beams (U.S. Pat. No. 4,030,831), measuring the doppler frequency shift of one laser beam relative to a reference laser beam (U.S. Pat. No. 3,950,100), and improving the signal-to-noise ratio in optical communication systems (U.S. Pat. No. 3,175,088). However, the prior art does not teach an optical heterodyne interferometric detection method or apparatus for the detection and adjustment of differences in the optical path lengths of two beams of electromagnetic radiation, such as two laser beams. Furthermore, the prior art does not teach an optical heterodyne interferometric detection method or apparatus for synchronizing a plurality of laser beams by adjusting the optical path lengths of each of the beams to equal the optical path length of a master laser beam.
Prior to this invention, timing of laser beams in fusion laser systems was accomplished by a device, such as a streak camera, to detect differences in optical path lengths between laser beams. For example, high speed electro-optical streaking cameras, such as the Hadland Imacon 675A, have been used to time beams in fusion laser systems. In operation the laser system is fired, and pulses from each beam are recorded on the streak camera in order to determine differences in the time-of-flight for an optical pulse in each of the beams. This technique is limited in precision by the resolution (10-15 picoseconds) of available streak cameras and the shortest pulses available from the laser system (generally in the 30-50 picosecond range). Another disadvantage of this technique is that a portion of the laser system must be fired in order to time the beams, and this is costly since it ages the system, and it is time-consuming since only one beam is timed per firing.